1. Field of the Invention
The present invention relates to a thermal flow sensor of a heating current detection type for detecting a flow rate of a fluid by electrically detecting the amount of heat transferred to the fluid from a heating element disposed in the fluid. In particular, the present invention relates to a thermal flow sensor and a method of generating a flow rate detection signal by the thermal flow sensor, which are capable of obtaining a detection value that differs depending on a flow direction of a fluid, with a simple configuration.
2. Description of the Related Art
In a typical thermal flow sensor, a heating element and a fluid temperature detector are disposed in a fluid, and a bridge circuit is formed by the heating element, the fluid temperature detector, and a plurality of resistors. A heating current to be supplied to the heating element is controlled so that the bridge circuit may always maintain a state of equilibrium. In this manner, the temperature of the heating element is always kept to be a temperature higher than the temperature of the fluid detected by the fluid temperature detector by a predetermined temperature.
By the way, there is widely used a thermal flow sensor of a so-called heating current detection type, which uses a heating current as a flow rate detection signal by utilizing the dependence of the heating current on the flow rate of the fluid. This thermal flow sensor, however, has a problem in that the flow rate detection signal takes the same value for normal flow and reverse flow, regardless of the flow direction of the fluid.
As a method for solving the problem, it is known to dispose two bridges upstream and downstream in the flow direction of the fluid and use a difference between respective heating currents as a flow rate detection signal, to thereby obtain a flow rate detection signal that differs depending on the flow direction of the fluid (see, for example, Japanese Patent Application Laid-open No. Hei 11-201793).
As another method, the following is known. A flow rate detection signal detected from a heating current is converted from voltage to current, and after a direction of the current is switched in response to an output from a fluid direction detection circuit for detecting the flow direction of the fluid, current-to-voltage conversion is performed, to thereby obtain a flow rate detection signal that differs depending on the flow direction of the fluid (see, for example, Japanese Patent Application Laid-open No. Hei 08-105779).
The conventional technologies, however, have the following problem.
In the methods disclosed in Japanese Patent Application Laid-open Nos. Hei 11-201793 and Hei 08-105779, a flow rate detection signal that differs depending on the flow direction of the fluid can be obtained, but both the methods need a complicated, large-scale circuit, which hinders reduction in size and cost.